Graft-versus-host disease (GVHD) limits the application of allogeneic bone marrow transplantation (BMT), a curative therapy for many hematological malignancies and inherited diseases. GVHD, when refractory to its first line therapy, steroids, leads to >70% mortality. A multitude of strategies have been tried to treat steroid refractory GVHD, but as yet the outcomes remain uniformly fatal. Current understanding of the biology indicates that GVHD severity is determined by the balance between cytopathic T effector cells (Teffs) and the cytoprotective regulatory T cells (Tregs) and that this balance is critically dependent on the level of inflammation. We recently demonstrated that human alpha1-antitrypsin (AAT), the major serum serine- protease inhibitor possesses, anti-inflammatory effects that were heretofore largely unrecognized. We have recently shown that AAT reduced several pro-inflammatory cytokines, positively modulated the Teff:Treg balance, prevented and treated GVHD in multiple murine models (PNAS, 2012). But the molecular mechanisms remain unknown. Preliminary data generated by us demonstrate a novel role for sialic acid- binding immunoglobulin-like lectins-G (Siglec-G), on host cells in mitigating GVHD (Blood, 2014). Unpublished preliminary data further suggest that Siglec-G is critical for the anti-inflammatory effects of AAT. In this proposal we will build on these exciting data to translate these observations into a proof of concept, first in human BMT clinical trial under an IND from FDA and meld it with further exploration of the molecular mechanisms of AAT mediated effects. The translational trial has recently been launched to assess the safety and efficacy of AAT in patients with steroid refractory GVHD (already accrued eight patients). Collectively, if successful, our proposal will lead to the development of an entirely novel therapeutic strategy while simultaneously providing novel biological insights into a fatal condition, steroid refractory GVHD. The specific aims (SA) of the proposal are: SA 1: Elucidate the cellular and molecular mechanisms of AAT mediated regulation of inflammation. In this SA we will test the hypothesis that binding of AAT to the inhibitory Siglec-G receptors is critical for regulating inflammation an the Teff-Tregs balance. SA 2: Perform a pilot clinical trial to determine whether administration of AAT to patients with steroid refractory GVHD will improve response rates. We will explore the hypothesis that administration of AAT will improve the response rates by day 28 in patients that are refractory to steroids GVHD.